The present invention relates to Spinal Cord Stimulation (SCS) systems and more particularly to a system and method for measuring the effectiveness of SCS system stimulation parameter sets (also known as protocols). An SCS system treats chronic pain by providing electrical stimulation pulses, through the electrodes of an electrode array, which electrode array is placed epidurally near a patient""s spine. The stimulation parameter set determines what stimulation pulses are used with the electrode array. The optimal stimulation parameter set for a specific patient may only be determined from empirical evidence of the stimulation parameter set""s success. Therefore, an effective method of measuring the efficacy of candidate stimulation parameter sets is an important tool in tuning the SCS system to achieve effective pain reduction.
Spinal cord stimulation is a well accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an Implantable Pulse Generator (IPG), electrodes, and electrode lead extensions connecting the IPG to the electrodes. The IPG generates electrical pulses that are delivered, through the electrodes, to the dorsal column fibers within the spinal cord. The electrodes are implanted along the dura of the spinal cord. Individual electrode contacts (the xe2x80x9celectrodesxe2x80x9d) are arranged in a desired pattern and spacing in order to create an electrode array. Individual wires, or electrode leads, connect with each electrode in the array. The electrode leads exit the spinal column and attach to one or more electrode lead extensions. The electrode lead extensions, in turn, are typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG is implanted.
Spinal cord stimulators and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940 that provides timed sequenced electrical impulses to a plurality of electrodes. As another example, U.S. Pat. No. 3,724,467 issued Apr. 3, 1973 for xe2x80x9cElectrode Implant For The Neuro-Stimulation of The Spinal Cord,xe2x80x9d teaches an electrode implant for the neuro-stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by leads to an RF receiver, which is also implanted.
In U.S. Pat. No. 3,822,708, issued Jul. 9, 1974 for xe2x80x9cElectrical Spinal Cord Stimulating Device and Method for Management of Pain,xe2x80x9d another type of electrical spinal cord stimulation device is taught. The device disclosed in the ""708 patent has five aligned electrodes which are positioned longitudinally on the spinal cord. Electrical pulses applied to the electrodes block perceived intractable pain, while allowing passage of other sensations. A patient operated switch allows the patient
Most of the electrode arrays used with known SCS systems employ between 4 and 16 electrodes. Electrodes are selectively programmed to act as anodes and cathodes, creating a stimulating group. The number of stimulation groups available, combined with the ability of integrated circuits to generate a variety of complex stimulation pulses, presents a multiplicity of stimulation parameter sets to the clinician. A known method of evaluating the efficacy of competing stimulation parameter sets is the walking test, wherein the clinician observes the patient walk before and after the therapy is applied. The effectiveness of the therapy is judged based on the clinician""s subjective observations of the patent""s gait, etc. This method clearly has the disadvantages of lacking an objective measure, and failing to provide an ongoing analysis of the efficacy of the stimulation parameter set once the patient leaves the clinician""s office.
A system and method for monitoring patient activity and adjusting stimulation parameters is described in U.S. Pat. No. 6,120,467. The system described in the ""467 patent computes both long term and short term averages of the filtered output of a sensor carried on a patient, which sensor is preferably an accelerometer. The filter filters out noise in the sensor output. The short term average is divided by the long term average to compute a normalized average. Thresholds are preset for rest, moderate activity, and vigorous activity levels. The normalized average is compared to the thresholds, and the amount of time that the normalized average fits each category of activity is stored. Any adjustments made by the patient to the therapy are also recorded and time tagged to allow time alignment with the activity recording. The stored values are later provided to a physician to provide an objective measure of stimulation effectiveness to compare with subjective evaluation provided directly by the patient. The physician may then objectively interpret the subjective patient information.
While the system and method taught by the ""467 patent may provide some advantages over a purely subjective approach, there are several disadvantages to the approach of the ""467 patent. The sensor data is filtered to remove noise, not to specifically pass data corresponding to selected activities. The normalizer may ignore long periods of activity, such as long walks, if the long term average is not substantially longer than the period of activity. Further, a period of very light activity, surrounded by periods of no activity, may be falsely normalized to a high activity value, Finally, the method of the ""467 patent relies on the assumption that a higher activity level is always an indication of improved stimulation. However, in many cases a patient may fidget due to discomfort, and thus the increased activity may be an indication of failure, not success.
What is therefore needed is a method of evaluating the effectiveness of candidate SCS stimulation parameter sets, wherein the evaluation method is objective in nature, is not subject to normalizer induced errors, targets specific physical activities, and continues to monitor the patient""s activity in the absence of visits to the clinician""s office.
The present invention addresses the above and other needs by providing an activity monitor for measuring the effectiveness of a Spinal Cord Stimulation (SCS) system. The activity monitor includes at least one motion or activity sensor which may be incorporated into a temporary stimulator, integrated into an implanted stimulator, or worn as a separate external stand-alone device. The output of the sensor is filtered to pass data associated with defined activities, and processed to measure the level of activity of a patient. Increased levels of targeted activities are an indication of successful pain management. Additionally, a walking gait monitor is used to evaluate the stability and smoothness of the patient""s walk as a further indication of successful pain management. In a preferred embodiment, the activity monitor provides feedback to an update stimulation parameters function to enable long term tuning of stimulation parameters.
In accordance with one aspect of the invention, there is provided an activity monitor which can be used with a temporary SCS system during the percutaneous trial. Such a temporary activity monitor allows the clinician to objectively evaluate both the applicability of SCS to a particular patient, and to compare the effectiveness of alternative electrode positions and stimulation waveforms.
It is a feature of the invention that the activity monitor used with the system may be integrated into an implantable SCS system to monitor pain management effectiveness over a long time period. Various stimulation parameter sets are available for execution once a permanent implantable stimulator is in place. Such stimulation parameter sets define various waveforms and combinations of electrodes. The ability to evaluate the effectiveness of a multiplicity of stimulation parameter sets, over a long period of time, permits a clinician to determine the best overall stimulation parameter set for a patient. And, as physiological changes take place over time, the system may continue to monitor the effectiveness of the pain management and provide the results to a clinician.
It is an additional feature of the invention to provide a temporary external device that may be strapped or otherwise carried on the body of a patient. Such a temporary device includes a motion sensor attached securely to the body of the patient to measure motion, a signal processor, and a recording device. The temporary device provides a capability to periodically reevaluate the effectiveness of an SCS system, which SCS system does not include an integrated monitoring system.
It is a feature of each of the above described embodiments of the present invention to filter sensor data to pass signals associated with activities of interest. The filtered sensor data is used to compute the variance of the output of a motion sensor as an estimate of the exertion level of a patient. Such a measure of exertion is an indication of the effectiveness of pain management because the presence of pain has the effect of causing a patient to minimize movement.
It is a further feature of the invention to analyze the motion sensor output to measure the stability and smoothness of a walking gait, or other identified activity, of a patient. Some types of pain are triggered by the act of walking, and a patient alters their walking gait to avoid such pain. The analysis of the walking gait provides a capability to detect such pain avoiding behavior.
It is another feature of the invention to identify the present activity of a patient, thereby allowing the system to limit data collection to defined activities, such as walking.
It is additionally a feature of the invention to provide a feedback mechanism for SCS systems to allow the systems to search for the most effective SCS stimulation parameter set for a patient. The SCS system measures the effectiveness of various SCS stimulation parameter sets over a period of time. The stimulation parameter set that provides the most effective overall pain management is selected for use. Later, as physiological conditions change in a patient, the SCS system can re-adapt to the developing environment and continue to offer effective pain management.
It is another feature of the invention to provide a patient override control. Such an override control provides the patient a means to immediately cancel the selection of a new stimulation parameter set, which stimulation parameter set may have resulted in unpleasant sensations or an increase in perceived pain.